by
James A. Fay, Dan Golomb and James Gruhl Energy Laboratory. Report No. MIT-EL 83-004
CONTROLLING ACID RAIN
James A. Fay, Dan Golomb and James Gruhl
Summary. High concentrations of sulfuric and nitric acid in raTn fn the northeastern USA are caused by the large scale combustion of fossil fuels within this region. Average
precipitation acidity is pH 4.2, but spatial and temporal
fluctuations of *1 pH unit have been observed. Amelioration of rain acidity requires significant reduction of precursor
emissions--the oxides of sulfur and nitrogen. Such reduction of emissions from existing sources will be difficult and
expensive. A pending legislativp proposal to reduce eastern U.S. emissions of SO2 by 10 Mty-' below the 1980 level of 22.5 Mty- 1 would reduce the acid sulfate deposition rate in the Adirondack mountains, an environmentally sensitive area, from 30 to 21.4 kg ha-ly-l. Based upon source/receptor
modeling, an equal reduction of Adirondack sulfate deposition rate could be achieved by a 7 Mty- SO2 emission reduction if the reduction is allocated according to source proximity to the sensitive area. The cost of reducing 10 Mty- 1 SO
2 emissions is estimated at $5 to $8 billion per year. Considerably lower costs could be realized in an emission
control scheme that is the more stringent the nearer the sources are to the environmentally sensitive areas, and if novel
approaches are implemented, such as seasonal or episodic emission reduction, NOx vs. SO2 emission control, emission redistribution, and least emission electricity dispatch.
Substantial emission reductions will probably be achieved in the more distant future by employing new combustion technology, such as lime injected multistage combustion, fluidized bed
combustion, coal gasifier combined cycle, magnetohydrodynamics, and- possibly others.
James A. Fay is Professor of Mechanical Engineering and Director of the Environmental Program, Energy Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139. Dan Golomb and James Gruhl are staff members at the Energy Laboratory.
Acid rain is a severe environmental problem, one that is difficult to ameliorate. The origins of precipitation acidity are well known--the oxides of sulfur and nitrogen formed in the combustion of fossil fuels.
By processes which are understood only in general but not in particular, these emissions are carried great distances, being transformed during passage to acidic species and then deposited in wet or dry form on the surface of the earth. Millions of tons of acidic material are thus
deposited in eastern North America each year, some of
it in
areas which
are so ecologically sensitive as to suffer damage to aquatic systems or
forest crops. The most direct attack on the acid rain problem involves
reducing significantly emissions of acid precursors--the oxides of sulfur
and nitrogen--by modifying the fuel or combustion process at the source.
Complete elimination of such emissions is out of the question and
substantial reductions, while technologically feasible, will be very
expensive in the aggregate.
The ecological damage from acid rainfall is not uniform throughout this region. Principally mountainous regions of eastern U.S. and Canada, called sensitive areas, are affected because of their limited capacity to buffer acid precipitation. Aquatic systems show most clearly such
effects, but forest species appear not to be exempt from damage. On the
other hand, crops grown
in predominantly agricultural regions seem not be
susceptible.
To provide some environmental relief to these sensitive areas,
Congress is considering
the imposition of sulfur oxide emission control
2
The more ambitious proposal, the Mitchell bill, would reduce annual SO2 emissions by 10 million tons, that is, by about 45 percent of the present emission level. It is estimated that the effect of such an emission
reduction on the average precipitation acidity would be an increase of pH (i.e., reduction of acidity) by 0.1-0.2 units above the current mean
value of 4.2. However, the annual deposition rate of sulfate will decrease commensurately to the emission reduction of SO2 . Whether environmental benefits are related to changes in pH or reduction of sulfate deposition rates, many sensitive areas would not be removed from the endangered list by this level of amelioration. Further reduction of SO2 and NOx emissions would be required to reduce precipitation
acidity to harmless levels.
While acid species are carried long distances from their precursor sources, a source closer to a sensitive are6 causes greater deposition than an equal source much further away. Such source-receptor
relationships, although presently not precisely defined, provide a basis for considering advanced control programs for SO2 and NOx emissions that would be more cost-effective. Such alternative strategies would concentrate the emission reductions among those sources closest to the sensitive areas. These cost-beneficial strategies become especially important for future reductions beyond those already contemplated, which may be more expensive and will undoubtedly involve balancing the relative costs and benefits of SO2 vs. NOx controls.
Even within the proposed control programs, but especially for more comprehensive ones, there are alternatives to the allocation of emission reductions which could decrease deposition rates or the cost of control. Interchange of high and low sulfur fuels, electric power generation and
transmission between regions close to and far from sensitive areas, least emission dispatch of electric power, and seasonal (summer) control of emissions have the potential for increasing the cost-effectiveness of control. Also, a different balance of SO2 and NOx reductions among such regions might be beneficial. The greater the degree of emissions reduction which is desired, the higher are the marginal costs of
reduction and the more varied are the methods needed and sources to be controlled.
The available methods for reducing sulfur emissions from existing sources consist principally of (1) removing sulfur from the fuel before combustion, (2) the removal of sulfur dioxide from flue gases, and (3) substitution of low sulfur for high sulfur fuel. But new technology
under development would remove sulfur during the combustion process or in a precombustion process, hopefully more cheeply and effectively. For reducing NOx emissions the principal method is combustion
modification. Presently practiced methods can be applied to existing plants by retrofitting, but developing technologies require new,
compatible boiler design, and even completely new plants which may be located elsewhere than the existing ones.
Gearing up for acid rain mitigation programs will require many years and enormous investments in supplying emission control devices and/or low
sulfur fuels. In the meantime, to offset the deterioration of the most threatened aquatic systems a program of liming of lakes, and possibly forested areas, may be needed. But there is little expectation that such palliatives, by no means inexpensive, can offset indefinitely present levels of acid deposition.
4
The Sources and Effects of Acid Rain
Precipitation acidity. While it has been known for more than a hundred years that rainfall sometimes is quite acidic, only within the past few decades has there arisen a recognition that acid rainfall deleteriously affects natural ecosystems, especially fresh water fish populations. Since World War II, first in Scandinavia and then in northeastern North America, monitoring of rainfall acidity and aquatic ecosystems has shown some evidence of increasingly severe aggregate effects. The acidity is principally associated with sulfate and nitrate ions in precipitation, although other, less prominent species may add to or subtract from the overall acidity. These principal ions are formed from the oxidation of sulfur and nitrogen oxides (SOx and NOx) in the atmosphere whose principal (but not sole) sources are the products of combustion of fossil fuels.
Part of the emitted gaseous SO2 and NO is oxidized to form the SO2" and NO3 ions which are deposited in raindrops. This
process is thought to be quite complex, involving several pathways of gas, liquid and solid phase oxidation. The presence of water vapor, oxidants such as OH and 03, and sunlight are considered to be essential in this transformation of primary to secondary air pollutants. After many hours and even days, the acid sulfates and nitrates so formed exist in the atmosphere as chemical components of very fine particulate matter, including liquid droplets. Some of this material can be incorporated into raindrops as they are formed or is scavenged from the air by rainfall originating at higher altitudes. Because these atmospheric processes are very slow, the precipitation of sulfate and nitrate ions
occurs to great distances from the point at which combustion effluent is injected into the atmosphere.
The acid content of rain is measured by its hydrogen ion
concentration. This concentration is defined on a logarithmic scale, each unit decrement in pH corresponding to a tenfold increase of hydrogen ion concentration. Pure water has a pH of 7. Rain water in equilibrium with the atmospheric carbon dioxide has a pH value of 5.6. A pH of 4 could be formed by one part per million of sulfuric acid or 2 ppm of nitric acid (or any proportionate combination of both). Typical rainfall pH, during episodes of noticeable acidtty, is in the range of 3.5 to 4.5. Hence acid concentrations in rain, on average, are about one part per million. On the other hand, the concentration of NOX and SOX in the atmosphere through which the rain descer-ds is about ten parts per billion. Because of chemical and physical factors, precipitation scavenges and concentrates the airborne species, delivering an acidic solution to the surface of the earth.
It is a common observation that rainfall is an episodic phenomenon, i.e., precipitation occurs during a small percentage of the time. The acidity of rainfall varies greatly from one rainfall event to the next, and even during a single rainfall. Peak hydrogen ion concentrations can be ten times higher than the annual average values. Thus much of the annual acidity deposited by rain may be contained in only a small
fraction of the total rainfall which falls during a very small percentage of the time.
Acidic rainfall is most noticeable near regions of heavy industrial activity, such as western Europe and eastern North America. Within such identifiable areas, estimates of sulfur sources show a ten-to-one
6
preponderance of man-made emissions over biogenic sources. In the U.S., about 90 percent of anthropogenic sulfur is released in the combustion of fossil fuels, about 2/3 of this by electric generating stations (Table 1). Of the total sulfur released into the atmosphere within the eastern U.S., about 20 percent ends up in acid precipitation and 80 percent in dry deposition and convective transport by the wind beyond the region's boundaries (1). (Of course, all of the emitted sulfur ultimately returns to the surface of the earth or ocean.)
Table 1. Distribution of U.S. SO2 and NOx emissions by source
category. From (2).
Category S02 (percent of tota) NO, (percent of total)
Utilities 65 31
Industrial boilers 12 21
Industrial prccesses 15 3.5
Transportation 3 40
Residential/commercial 5 3.5
Solid waste disposal 0 0.5
Mi scel 1 aneous 0 0.5
Total 100 100
i
i-A similar proportioning of sources and sinks also holds for nitrogen oxides. Of the combustion sources of N0x, vehicles (principally the automobile) account for nearly half. In terms of acidifying potential, aggregate NOx emissions constitute about one-third of the total and
different at the point of acid precipitation since the source streams are depleted at different rates.
Over the past few years, high quality precipitation chemistry monitoring stations were established across North America. These
stations measure precipitation acidity (hydrogen ion concentration) and other important cations and anions in rain, snow, dew, etc. Fig. 1 shows the data obtained in 1980 for hydrogen ion deposition rates (2). The units are millimoles m-2 y - 1, obtained by multiplying the mean
concentrations in precipitation by the amount of precipitation at the site. The discrete numbers are the records from individual stations; the contours (isopleths) are hand-drawn and therefore somewhat subjective.
From measurements of this type it has been round that hydrogen ion deposition reaches a maximum in the Ohio-Pennsylvania-West Virginia triangle, but depositions in excess of 50 mmoles m2y- 1 (equivalent to an annual average pH of 4.3, assuming 100 cm y-1 precipitation)
occur as far west as Illinois, north to southern Ontario and Quebec, east to the Atlantic seabord, and south to North Carolina and Tennessee.
Relating emissions to deposition. Quantitative estimates of the relation between emissions of acid rain precursors and amount of acid deposition are based upon mathematical models of the physical and chemical processes of atmospheric transport, transformation and
deposition. There is a great variety of such models--a recent study enumerates 43 of them--some of which are very complex. As the detail of description in time and space generated by a model increases, so does its complexity, cost of computation and the degree of detailed specification of the meteorological, chemical and physical processes believed to be at work. Current studies of model development seek to improve the
Fig. 1. Precipitation-weighted hydrogen ion deposition in eastern North 2 -1
America (mmoles m- Y). Crosses mark box area discussed later. From (2).
reliability of models through modifications which increase the correlation between measured and calculated variables, such as acid deposition rates, etc.
A nearly universal and simplifying assumption of acid rain modeling is that of linearity, that is, all physical and chemical processes (e.g., dry deposition, oxidation of precursors) proceed at a rate proportional to the concentration of the pollutant in question. When this is so, the effects of pollutants emitted from each source may be added together at each receptor so that the apportionment of the effects produced at the
receptor may be allocated quantitatively to the various sources, no matter how distant. But where processes are not linear, the linearity hypothesis will only approximate the true rates and the resulting
conclusions regarding acid deposition may not be accurate. Only further research of the physicochemical processes and nonlinear modeling can resolve whatever uncertainties now exist regarding the linear hypothesis.
The simplest model is the chemical mass balance or box model. For example, consider the atmosphere which covers the U.S. east of the Mississippi as a box into which S02 and NOx emissions from all
sources are introduced at a constant averagie rate. Since the mass of elemental sulfur and nitrogen is. conserved, these species are lost at an equal rate from the box because of wet and dry deposition to the earth's
surface or by net advection by the wind out of the boundaries of the box. Using atmospheric measurements one can estimate the various components of this mass balance and thus determine in what form and proportions the emissions leave the box. The box model provides the
simplest argument which relates acid rain deposition to anthropogenic emissions of SO2 and NOx .
10
Neither acid rainfall nor precursor emissions are uniformly
distributed throughout such a large region. ,To obtain a more detailed understanding of the mass balance, the region can be subdivided into a large number of microscopic box models--say 50 x 50 km in base area. By
performing simultaneous mass balances in all of these small elements, a much more detailed description of acid deposition within a broad region can be obtained. But because the relevant atmospheric monitoring has been carried out in only a few of these microscopic boxes, the
atmospheric processes of transport, transformation and deposition must be modeled mathematically in each box in order to accomplish an overall mass balance. Since knowledge of these processes is only approximate,
uncertainties about the model predictions are thereby introduced. Episodic models seek to describe the daily changes of precursor behavior and acid rainfall through a detailed tracking of the
meteorological events which govern the fate of pollutant between source and receptor. Because of the great expense of such modeling, attention is first focussed on the explanation of high pollutant episodes lasting several days to test the performance of such models.
One method of reducing the complexity of acid rain modeling is to average the mass balance over a long time period--say a month, season or year. The resulting models are called climatic or statistical models.
The climati models can provide average acid deposition rates without requiring detailed information of meteorological events on a diurnal time scale. Time-averaged deposition may prove to be an acceptable surrogate for evaluating environmental effects and the corresponding benefits from mitigation strategies.
ecosystems are adversely affected by the acidification of pond or lake water, but in ways which are also determined by the kind and proportions of anions and cations rather than simply by the net hydrogen ion
concentration. In many cases precipitation enters a lake principally via a drainage basin where the acidic composition may be altered. Where it exists, the buffering capacity of the drainage basin and lake bed can substantially modify the acidic quality of the rainfall and prevent the increase of lake or pond acidity. Given the complex chemical processes at work in aquatic ecosystems, it is difficult to predict in detail how they would respond to changes of precipitation acidity.
Many lakes in the northeastern U.S. and eastern Canada have been markedly affected by acid precipitation because of the lack of natural buffering capacity, the underlying geologic structure being granite
rather than limestone. Because acid rainfall is widespread, sensitive receptor areas can be identified by surveying for the early signs of ecosystem deterioration among the aquatic ecosystem populations. It is these sensitive areas which would be the principal beneficiaries of reductions of acid precipitation.
The damage to forests from acid rain is less certain and
quantifiable. Soil characteristics and tree species will determine the effects. Given the ubiquity of silviculture in the northeastern and southeastern U.S., even minor damage caused by acid rain could aggregate to significant economic impacts.
Galloway and Cowling (3) defined the acid-susceptible areas of North
America in terms of surface waters contained in igneous and metamorphic rock. Their sensitivity map is reproduced in Fig. 2. Omernik and Powers (4) developed a new sensitivity map based on total alkalinity of surface
Fig. 2. Regions of North America (shaded) containing lakes sensitive to
waters. This map permits a better resolution for identifying watersheds that are susceptible to acidification than is shown in Fig. 2.
The Impact Assessment Interim Report produced under the auspices of the United States-Canada Memorandum of Intent on Transboundary Air
Pollution (5) identified the following watersheds that might be sensitive to acidic deposition:
o Northern Michigan, Wisconsin, Minnesota o New England
o Adirondack region of New York o Southeastern New Jersey
o Western North Carolina and parts of the Smoky Mountains o Southeastern Ontario
o Southeastern Quebec
o Maritime Provinces and Newfoundland
The U.S./Canada report cautions that "a more comprehensive and verifiable estimate of acuatic acidification carrying capacity must await the
findings of research presently being conducted."
In addition to identifying areas sensitive to acid deposition, it is important to relate them to present deposition patterns. Obviously, only sensitive areas that are exposed to excessive acid loading will become acidified. However, there is no unambiguous criterion of what
constitutes an excessive loading and it is not even clear whether
sulfate, nitrate or hydrogen ion loading (or a combination of these ions) causes the ecological damage. Recent assessments (5,6,7) indicate that
sulfate deposition may be a good indicator for environmental damage. They suggest that at a sulfate loading of 15 kg ha- 1 y-, only the most sensitive lakes become acidified; at a loading of 45 kg ha-1
14
y-1, moderately sensitive lakes also become acidified, and sensitive
species become extinct. Using a 30 kg ha-1 y- threshold, the exposed sensitive areas are confined to upper New York state, New England, southern Ontario and Quebec and parts of the Smoky Mountains.
Emission Reduction/Acid Deposition Relationship
Box model. Simple acid rain models may be used to estimate the
effects on acid deposition of changes in emission rates of the precursors
SO2 and NOx . The commonly used linear model predicts that the acid deposition effects of many sources is simply the sum of the effects from each source considered separately. The simplest of linear models is the -mass-balance-in-a-box model described by Golomb (1) and summarized here.
Let us take a box of sufficient size such that most of the transport and transformation processes occurs within the box. We account for all sources in the box: emissions and influx from outside the box, and all sinks: deposition (wet and dry) and efflux through the walls. The
"sufficient" size stipulation is only necessary to ensure that the influx is small compared to the emissions.
The northeast quadrant of the U.S. was selected as a suitable box. The boundaries run from the SW corner of Tennessee to the NW corner of
Illinois, east to the NE corner of New Hampshire, south to the SE corner of North Carolina, and back to Tennessee. The box is marked with crosses in Fig. 1. The bottom area of the box is 1.2 x 106 km2. For a
steady state to be established, a sufficient time period needs to be chosen, say one year. The year 1978/79 was analyzed. The emissions within the box are 2.2 x 1011 moles y-1 of SO2 and 1.8 x 1011
moles y-1 of NOx, with an estimated uncertainty of *10 percent.
The wet-deposition of sulfate, nitrate and other ions inside the box was analyzed by Pack (8) based on the MAP3S and SURE monitoring network
data. Integrating the Pack isopleths (which are similar to Fig. 1), the total annual wet-deposition of sulfate amounts to 4.3 x 1010 moles
-110-y , and nitrate 3.9 x 10 moles y-1 (with an uncertainty of *25 percent).
Thus, it is found that 19 percent of the emitted SO2 is
wet-deposited as sulfate within the box, and 20 percent of the emitted NOx Is wet-deposited as nitrate. The rest evidently follows other pathways such as dry deposition and net efflux. Similar results for eastern North America were obtained by Galloway and Whelpdale (9), and for western Europe by Garland (10) and Rhode (11). The average
concentration in precipitation is 27 pmoles 1-1 of sulfate and
24 pmoles 1-1 of nitrate. If every sulfate ion carried two hydrogen ions and ever) nitrate one, the average H+ concentration would be 78 amoles 1-1, pH = 4.1. However, some neutralization occurs due to the presence of ammonium, calcium and magnesium ions and the corrected average H+ concentration is 67 umoles 1-1, ,H = 4.2 (1). This is in good agreement with the mean of observations (12). Note that the calculated values are averages; local and temporal fluctuations by a
factor of 10 in hydrogen ion concentrations (pH *1.0) have been observed (12). Sulfate/nitrate ratios are also subject to spatial and temporal variations. However, on the average, 2/3 of the precipitation acidity is due to sulfuric acid and 1/3 to nitric acid.
Proportionate emission reduction. The simplest case to consider is an across-the-board reduction of SO2 and NOx. If both emissions and
16
depositions are averaged over a large geographic area, such as the eastern U.S., then the spatially averaged deposition rates of each species would be proportional to the aggregate emission rates, assuming that the latter are reduced in a geographically uniform way. For
example, a tenfold reduction of emissions would result in a tenfold
decrease of wet-deposited ions everywhere within the zone of influence of the sources whose emissions are reduced. Of course, across-the-board
reductions are impractical to achieve, especially for NOx, and certainly not tenfold.
Table 2 lists the effect of proportionate emission reduction on the annual average rain acidity by successive elimination of the source categories of Table 1. From Table 2 it becomes evident that significant
improvement of the average acidity can only be achieved by very
substantial curtailment of emissions. Since such area-wide curtailments may not be technically and economically feasible, a more effective
approach, at least in the near term, would be the reduction of emissions primarily in source areas that can be shown to &ontribute most of the acid deposition to selected sensitive areas.
Source apportionment modeling. To provide alternative approaches, it is necessary to resort to a particular model which takes into account the fact that a more distant source has a lesser effect than a nearby source on the amount of acid deposition, other things being equal. Using the climatic model of Fay and Rosenzweig (13), Table 3 lists the percent contribution to acid sulfate in rainfall at a receptor in the Adirondack Mountains caused by the aggregate SO2 emissions in each of the states
elimination of emissions from source categories Emission reduction
(percent below present) SO2 NOx 100 0 31 0 52 95.5 100 Precipitation acidity* [H+3 A moles 1-1 67.0 38.2 31.1 32.9 21.1 7.0 2.5 pH 4.2** 4.4 4.5 4.5 4.7 5.15 5.6
1. The present estimated and observed average acidity in the northeastern U.S.. 2. Elimination of S02 emissions from all power plants.
3. Elimination of SO2 and NOx emissions froi power plants.
4. Elimination of SO2 emissions from power plants and industrial boilers. 5. Elimination of SO? and NOX emissions from power plants and industrial boilers. 6. Elimination of emissions from all major industrial sectors and transportation. 7. No emissions whatsoever, the acidity of water in equilibrium with CO2 (carbonic acid).
*Probable error bound *30 percent of [H+], =0.15 pH.
**Large local and temporal fluctuations have been observed; factor of 10 in [H'], *1.0 pH. Case (1)
(2)
(3) (4)(5)
(6) (7)18
Table 3. Relative contributions of U.S. sources to acid sulfate in rainfall at Adirondack receptor
Contribution (percent of total) Rank by source strength Pennsylvania Ohio New York Indiana West Virginia Michigan Massachusetts Illinois Kentucky New Jersey
Other eastern U.S. states Total*
*The observed annual average acid sulfate deposition at Whiteface Mountain in the Adirondacks in recent years is about 30 kg ha-ly- 1
(12). Of this, U.S. sources contribute about 70 percent, Canadian sources about 30 percent (5).
State 17.4 14.8 10.5 7.6 6.6 5.2 4.1 3.9 3.4 2.9 23.6 100
(Fay and Rosenzweig's model allows the estimate of ambient sulfate concentrations; in these calculations we assume that sulfate
wet-deposition is proportional to ambient sulfate concentrations.) The ten largest contributors aggregate to more than three quarters of the total effect. Also shown is the rank order of the source strengths of each state. Obviously some of the nearby states are greater contributors than would be expected by their source strengths.
The source/receptor relationships listed in Table 3 are those for a particular model. Other models give similar but not identical results.
It is hoped that further development of models will provide more reliable estimates of the apportionment of acid deposition at a receptor among the various sources of precursor emissions.
Of particular interest is the relative contribution of U.S. and Canadian sources to acid deposition in the northeastern U.S. and eastern Canada. The U.S.-Canada joint study (5) estimates that more than 30 percent of acid sulfate deposition at an Adirondack receptor originates in Canada. In another study by Galloway and Whelpdale (9), the fractions of domestic emissions which are "exported" to the neighboring country were found to be 14 percent and 33 percent for the eastern U.S. and
Canada, respectively. But because Canadian direct emissions are only 15 percent of U.S. emissions, the flow into Canada exceeds that in the reverse direction by a factor of three. However, there is no estimate yet of the percent of Canadian acid rainfall which is due to U.S.
sources, and vice versa. Thus the amount of acid deposition which might be mitigated by emissions reduction in both countries is as yet uncertain.
Pending legislative proposals and alternatives. Congress has been considering two major bills for reducing sulfur emissions so as to
20
mitigate rain acidity. Both propose substantial reductions within a 31 state region east of or bordering on the Mississippi River to be
accomplished within ten years. Reduction allocations among the states are based upon 1980 emissions from utility plants. The Mitchell bill would require a reduction of SO2 emissions of 10 million tons per year,
-1
while the Moynihan bill would result in a reduction of 8 Mty-1 to be accomplished within 10 years after the enactment. Since the 1980
emissions in this 31 state region are estimated at 22.5 million tons per year, these are reductions of 45 percent and 36 percent respectively. The formula for allocation of these aggregate amounts among the states is different for each bill, but places the greater burden on states which emit, on average, sulfur oxides in excess of 1.2 pounds of SO2 per
million Btu of fuel heating value. Most of the abatement penalty will fall on states which use extensively high sulfur coal in utility
boilers. These reductions are presumed to be achievable by substituting low sulfur fuel for currently used high sulfur coal or by desulfurization of flue gases from large sources, principally electric utility plants.
Table 4 compares the distribution of 1980 sulfur emissions among the states, arranged in rank order, with the distribution of reductions required by the Mitchell and Moynihan bills (14). The ten highest
emitting states contribute 68 percent of the total emissions from the 31 state region, but would be required to contribute 79 percent and 91 percent of the reductions, respectively. Thus neither bill requires a uniform percentage reduction of emissions among all of the states, but allocates reductions principally to the major emitters, especially so for the Moynihan bill.
State Ohio Pennsylvania Indiana Illinois Missouri Kentucky West Virginia Tennessee New York Fl ori da Other (21) All states Emission (1980) (percent (kty-1) of total) 2800 12.5 2165 9.6 2080 9.2 1580 6.8 1350 6.0 1100 4.9 1100 4.9 1085 4.8 1080 4.8 985 4.2 7275 32.3 22500 100 Emission reduction Mitchell Moynihan (percent below 1980) 58 53 10 36 57 56 50 41 68 56 63 71 53 51 62 70 21 7 36 29 29 9 45 36
Table 5. Effect .of proposed sulfur emission reduction on sulfate deposition at Adirondack receptor
Emission
1980
Deposition*
1980 Mitchell Moyni han (percent of total) (percent of total) (percent of 1980 total) Ohio Pennsyl vani a Indiana Illinois Missouri Kentucky West Virginia Tennessee New York Florida Other (21) All states Emissions (percent of 1980)
*Excludes Canadian contribution to deposition. State 12.5 9.6 9.2 6.8 6.0 4.9 4.9 4.8 4.8 4.2 32.3 100 14.8 17.4 7.6 3.9 2.1 3.4 6.6 2.6 10.5 0.9 30.2 100 6.3 10.4 3.2 2.0 0.7 1.3 3.1 1.0 8.3 0.6 22.2 59.1 6.9 11.2 3.3 2.3 1.0 1.0 3.2 0.8 9.8 0.6 27.1 67.2 55.2 63.6
---distributed among the sources, sulfate deposition rates may not be reduced in proportion to the aggregate emission reduction. We may use source apportionment models to estimate how the emissions reduction will affect the deposition rates at receptors located in sensitive areas. In Table 5 we show such a calculation for a receptor located in the
Adirondacks based upon the model of Fay and Rosenzweig (13). In the first two columns are shown, respectively, the distribution of 1980 sulfur emissions and sulfate depositions caused by these emissions
(excluding Canadian contributions). We note that the nearby sources contribute, in proportion to their strength, more than the distant sources--especially New York. In the third and fourth columns we have determined the net deposition rates after applying the emission reduction requirement of the Mitchell and Moynihan bills, respectively. It can be seen that the aggregate decrease of the deposition rate in either case is only a few percentage points above the aggregate reduction of emissions,
both normalized to their 1980 values. Thus, in spite of the geographic nonuniformity of emission reduction, the aggregate deposition rate is
quite commensurate to the aggregate emission reduction in the 31 states. The measured sulfate deposition rate at the Adirondacks is about 30 kg ha'ly-1 (5). Of this, 21 kg is estimated to be due to U.S. sources, 9 kg to Canadian sources. The Mitchell bill would reduce the U.S. contribution by 41 percent, to 12.4 kg ha-ly -1. The average
annual rain acidity would be expected to increase from the present pH 4.2 to 4.35.
The allocation of emission reduction under these bills is not as effective as it might be if the distances between source states and target receptor were taken into account. For example, New York and
24
Tennessee have equal 1980 emissions but Tennessee's contribution to the Adirondack deposition is only 35 percent that of New York. Yet under the Mitchell bill Tennessee must reduce its emissions by three times as much as New York. Since it is four times more effective to reduce emissions in New York than Tennessee, at least as far as Adirondack depositions are concerned, it would be logical and probably more cost-effective to
require a greater reduction for New York and a lesser reduction for Tennessee than stipulated by the Mitchell bill. This would decrease the aggregate emission reduction but not increase the Adirondack deposition rate.
To illustrate such an alternative, we have calculated an allocation of emission reduction which would achieve the same Adirondack deposition rate as the Mitchell bill and which would conform to one of two
requirements: (1) emissions of any state shall not exceed 1980 levels, and (2) for each state the contribution to Adirondack deposition per million Btu of fuel heating value shall be the same. Very distant states
(e.g., Louisiana, Arkansas) will not have to reduce emissions at all. States very close to the Adirondacks will have to reduce emissions more than is required by the Mitchell bill. The second requirement would maintain a constant ratio of environmental cost (deposition rate in sensitive area) to local benefit (fuel heat consumed), an equitable criterion for allocating emissions.
The results of such an alternative allocation are shown in Table 6 and compared to the emission reductions of the Mitchell bill. This alternative exempts 12 distant states from any emission reduction below 1980 levels. Of the 10 largest sources, 6 would have smaller and 4 greater reductions than the Mitchell bill requires. In the aggregate,
State Ohi o Pennsylvania Indiana Illinois Missouri Kentucky West Virginia Tennessee New York Fl ori da Other states (21) All states Emission reduction Alternative Mitchell (percent below 1980) 59 58 62 40 40 57 0 50 0 68 39 63 60 53 22 62 59 21 0 36 35 29 32 45
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the alternative allocation requires a 32 percent reduction of 1980
emissions while the Mitchell bill specifies a 45 percent reduction. This translates into an aggregate emission reduction in the 31 states of 7 Mty-1 vs. the 10 Mty-1 of the Mitchell bill, yet the environmental effect in the Adirondacks would be the same.
There are, of course, other sensitive areas which might be affected differently than the Adirondacks. Also, allocation of emission reduction based upon other criteria, such as the cost of reduction, might lead to different relations between emission and deposition reductions than the above. Furthermore, trading of emission reductions between states, as is contemplated by the Mitchell bill, will have some differential effect upon deposition rates. All of these aspects deserve more careful
scruti ny.
Redistribution of emissions. Atmospheric models help to identify source regions which contribute the bulk of acidity to ecologically sensitive areas. Conversely, these models also delineate regions whose emissions are unlikely to be conveyed to sensitive areas. It may be asked whether such non-contributing regions could possibly increase their emissions above the present level without materially affecting sensitive areas. In such a fashion, the present aggregate emission level in the U.S. could be kept constant, with some regions experiencing an emission reduction and others an increase.
An emission redistribution could be accomplished by a fuel exchange scheme. Acid rain source regions would import low sulfur coal (or other low emission fuel), and export high sulfur coal to non-source regions. In such an exchange, the socio-economic impact on high sulfur coal
Exchanges of coal for liquid fuel should also be considered. Schemes can be envisioned whereby low sulfur coal displaces high sulfur coal in power plants near sensitive areas, high sulfur coal displaces oil and gas in power plants in non-source regions, and the displaced oil and gas is used locally or elsewhere for internal combustion engines and petrochemicals, thereby cutting our dependence on imported oil. It should be noted that plants designed exclusively for combustion of liquid fuels (oil or gas) cannot switch to coal without major reconstruction of the boiler unit. Coal substitution will become attractive when the existing plant nears retirement or when liquid fuels near depletion, whichever comes first.
Emission redistribution could also be achieved by long distance transmission of electricity. High sulfur coal could be burned in
non-source region power plants and electricity could be transmitted to source region users. In this scheme, the existing power grid would be used to full capacity as well as possible new high voltage (direct current) long distance transmission lines.
It should be recognized that an emission redistribution scheme is contrary to present air quality management practice under EPA's new source perforTance standards (NSPS). Substitution of coal for oil
categorizes the plant as a new source. Present NSPS regulations require 90 percent sulfur removal from high sulfur coal and 70 percent removal of low sulfur coal; i.e., a scrubber installation is mandatory everywhere. For plants that substitute one kind of coal for another, most state implementation plans (SIP) require an emission limit of 1.2 lb SO2 per MBtu, ruling out the use of high sulfur coal without scrubbing. An emission redistribution scheme might require non-uniform NSPS and SIP variances tailored to meet acid rain environmental goals.
28
Seasonal and intermittent emission reduction. The concentration of acidic matter in precipitation and its deposition rate is highly variable over time and space. However, the bulk of sulfate and hydrogen ions appears to be deposited in the spring and summer months (12,15). Fig. 3 shows the monthly wet deposition of ions at Brookhaven, NYfrom which we estimate that 53.5 percent of the annual total acid deposition falls in the months June-September and 70 percent in May-October. If a similar distribution occurred in the sensitive areas, the annual acid load could be reduced significantly if low sulfur coal would be substituted for high sulfur coal in the summer months, especially in the source regions. Such seasonal fuel switching would also lessen the adverse socio-economic impact on existing coal mining districts that a year-round substitution would entail.
Intermittent emission control during pollution episodes is another possibility for reducing total acid deposit'on. Evidence is mounting that acidic metter is deposited most heavily following persistent elevated pollution episodes (PEPE). These episodes occur mainly in summer as a consequence of stagnating anticyclonic systems located over eastern North America. This system causes E recirculation of air over the emitting areas with gradual build-up of pollutant concentrations. The phenomenon is manifested by the appearance of haze which can blanket much of the eastern U.S. and Canada. The episode is usually terminated
by passage of a cold front or a possible squall line preceding the cold front. The pollutants are effectively scavenged by the ensuing
precipitation or thundershowers. If criteria for predicting these episodes were firmly established, emissions could be curtailed at their onset (PEPE alert) and consequently high acid precipitation episodes as
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30
well as the blanketing haze could be averted.
The pending legislations and proposals are primarily aimed toward SO2 emission control. However, it is also important to consider possible NOx emission control. Since NOx emission control technology
may in fact be cheaper and simpler than SO2 control, any effective acid rain control strategy should include NOx vs. SO2 emission control
trade-offs.
Controlling Emissions
Massive emission reductions such as required under the Mitchell and Moynihan bills are presumed to be accomplished by installation of
emission control technology or by substitution of low-emission fuel. The following methods would also be authorized under the pending
legislation: least emission dispatch, early retirement of sources, energy conservation, interstate trading of emission reductions, and precombustion cleaning of fuels (16). We shall briefly discuss some of these alternatives, their potentials, prospects, and costs.
Flue gas desulfurization (FGD). There is a voluminous body of
literature but meager commercial experience with FGD devices (17,18). In 1979, only 7 percent of the U.S. electric generating plants were fitted with FGD (7).
The basic operative mechanism of FGD is to bring all the
post-combustion flue gas in contact with an aqueous solution or slurry of finely ground limestone or lime, with occasional additives of other
alkaline substances. This is the so-called wet scrubber method. The alkaline solution/slurry reacts with SO2 to form a sludge of calcium
sulfite and sulfate (gypsum) which accumulates in the bottom of the reactor. Disposal of the sludge causes considerable engineering, land-use and environmental problems. There are recovery processes available that can regenerate the original sorbent from the spent waste, but their costs are so high that wet scrubbers producing a throwaway sludge are preferred.
Dry scrubbers have been demonstrated but have not yet been used on a commercial scale. Dry scrubbers may cost less and be more suitable in water-deficient areas. Their SO2 removal yield may be lower than that of wet scrubbers.
Low sulfur coal substitution. Lower sulfur emission rates could be achieved by using low sulfur coal (LSC). Eastern high sulfur coal (HSC) in the average contains 2.7 percent sulfur, western coal 0.7 percent. Limited LSC is also available from eastern coal mines but is used primarily for metallurgic purposes. Western LSC has a 15-25 percent lower calorific content than eastern and midwestern coal, so larger
quantities of LSC need be consumed to produce the same amount of heat (or electricity). About 26 Mt of 2.7 percent S coal need to be replaced by 31 Mt of 0.7 percent S coal to achieve a 1 N:t SO02 reduction.
In general the price of western LSC delivered to an eastern plant is
50 percent greater than eastern HSC. The price differential is even greater for a mine-mouth plant or one holding a long term (low price) contract for HSC. In addition to the price differential, other major barriers to the replacement of HSC by LSC are (a) capital requirements for developing new LSC mines, (b) lack of adequate transport facilities, (c) plant modifications needed to burn lower heat coal, (d) breach of long term contracts, and last but not least (e) the socio-economic
32 disruption of eastern coal mine districts.
Physical coal cleaning (PCC). There are two basic types of coal cleaning, chemical and physical. Chemical cleaning uses a solvent for extraction of sulfur and could be successful in removing 40 to 60 percent
(up to 80 percent in some advanced solvent refined processes) of sulfur from the coal. Chemical cleaning processes are neither commercially available nor economically competitive with other types of sulfur
removal. Thus, when coal cleaning is mentioned as an acid rain control method, usually physical coal cleaning is implied.
Physical coal cleaning consists mainly of coal washing by water. Coal washing can remove between 15 and 35 percent of the coal's sulfur. Pulverized coal can be further cleaned using a froth flotation process that takes advantage of the hydrophobic properties of coal and the
hydrophilic properties of the impurities. In fact, between 20 to 75 percent (in the Ohio River Basin between 50 to 100 percent) of the coal used by electric utilities is already cleaned, because it reduces the transportation cost, reduces the amount of ash, decreases boiler and flue duct fouling, and reduces the cost of pulverizing coal at the plant. Some studies indicate that over 1 Mty- 1 SO2 additionally could be eliminated by PCC (19).
Advanced technology. There are several methods under development that may reduce both SO2 and NOx emissions more effectively and at a lower cost than available technology. One of the most promising and least costly methods may be the lime injected multistage burner (LIMB). In this method, lime (or limestone) is injected directly into the combustion chamber where it may absorb up to 40-50 percent of the SO2.
NOx emissions. The cost of removing one ton of SO2 by LIMB may be
considerably less than by FGD. A related method is the fluidized bed combustor (FBC) where the crushed coal is burned in a bed of crushed limestone suspended by air blown in from below. The limestone "picks up" the sulfur, and the relatively low combustion temperature keeps down the NOx emissions.
Probably the main reason that LIMB and FBC technology is less advanced than FGD is the perception in industrial circles that these methods will not meet regulatory requirements (e.g., NSPS). A
comprehensive and cost-effective acid rain control strategy should foster the development of advanced emission control technology. For even if the SO2 removal efficiency, say of LIMB, at an individual source is smaller than of FGD, the aggregate emission reductions and cost savings that can be achieved by widespread application of these techniques may be far greater.
Other techniques, still in the early stages of development, have the promise of reducing SO2 and NOx emissions while improving the overall efficiency of electric power generation. Ariong these are coal gasifier combined cycle and magnetohydrodynamics but it is doubtful that these techniques could contribute significantly to an acid rain control program within the next two decades.
Least emission dispatch (LED). When decreased acid deposition is desired, e.g., during high pollution episodes, the electric generating plants with highest emissions could curtail output or shut down
completely while those with lesser emissions (e.g., oil- and gas-fired power plants) take over the excess load. LED is exactly the converse of the generally practiced least cost dispatch, for usually the highest
34
emission plants (i.e., those using high sulfur coal) generate cheaper electricity. However, LED may in the aggregate be cheaper than
installing emission control devices on HSC power plants. In a way, LED is similar to the emission redistribution scheme described above: higher emissions could be permitted in areas and periods that do not contribute significantly to acid deposition in the sensitive areas. The emission redistribution could be accomplished by integrating several utility systems over a regional network, and reducing the load on those units that contribute most severely to acid deposition.
NO, emission control. Pending legislation is primarily concerned with reducing SO2 emissions. Since on the average 1/3 of the deposited acid is nitric acid (in some seasons, e.g., winter, and localities, e.g., near metropolitan/industrial areas, the proportion of nitric acid can be even larger), the control of NOx emissions should not be neglected. In fact, controlling NOx may be easier and less costly than controlling SO2. For most of the sulfur is chemically bound to the fuel, while NOx is primarily formed in the high temperature combustion process by
recombining the nitrogen and oxygen of the air. Combustion
modifications, such as adjusting the fuel to air ratios, and staged combustion (e.g., the stratified charge internal combustion engine) have shown to be able to reduce NOx emissions by 40-60 percent (18), and most importantly, could achieve such emission reductions without major modifications or replacement of existing facilities.
Conservation. A surefire acid rain control measure is conservation. Every kWh saved, besides bringing economic advantages, puts 10-20 g less SO2 and 5-10 g less NO through the smokestack of an eastern
Cost assessment. For a reasonably accurate cost figure of any acid rain control strategy one must perform virtually a case-by-case analysis of every major emission source. Some plants are retrofittable with FGD devices, other cannot be so modified for lack of space or disposal
areas. Some plants are accessible to LSC, others may be located at a HSC mine mouth. Also, for a reasonable cost assessment, the proper mix of control strategies, such as emission redistribution, least emission dispatch, seasonal or episodic emission controly SO2 vs. NOx emission control, etc. need be considered.
To obtain a very approximate estimate of the costs, let us assume that the Mitchell bill goal of reducing SO2 emissions by 10 Mty- 1 in 31 states within 10 years would be accomplished with the following mix: 45 percent FGD, 45 percent LSC substitution, and 10 percent PCC.
A survey (17) of 26 operational FGD systems in U.S. coal-fired power plants with average capacity of 360 MW revealed that the mean capital cost (in 1983 dollars) for new installations is $150/kW with a standard deviation (s.d.) of $30/kW. Capital costs include cost of equipment, installation, site development, and start-up. For retrofit, the mean cost is $180/kW, s.d. $60/kW. The annual operating cost for new
installations is 10.2 mills/kWh; for retrofit 11.4 mills/kWh (s.d. 3 mills/kWh). Annual costs include cost of raw materials, utilities, operating labor, degpreciation, replacement of parts, and interest on borrowed capital.
Let us assume the following parameters: sulfur content in coal 2.7 percent, heat content 12,000 Btu/Ib, plant thermal efficiency 0.35, FGD SO2 removal efficiency 0.8. Thus 1 kWe h removes 15.4 g SO2 . At
Mty
-The cost of substituting LSC for HSC varies greatly from plant to plant depending on the cost of transport and the current price the plant pays for HSC. A very rough price estimate of HSC delivered at an eastern plant is $30 per ton; LSC is $45 per ton. To remove 4.5 Mty- 1 S02
117 Mty- 1 HSC (2.7 percent S) need be replaced by 139.5 Mty-1 LSC
(0.7 percent S). The price differential is about $2.8 billion per year not counting possible capital requirement of rail and storage facilities.
Physical coal cleaning costs vary substantially depending on sulfur content and other characteristics of the coal. Costs per ton SO2
effectively removed (after combustion) range from $310 to 430 for eastern Midwest coals, $430 to 720 for northern Appalachian coals, and over $1000 per ton for Alabama and southern Appalachian coals (19). Assuming an average price of removing 1 ton SO2 by PCC of 1400, the cost of
removing 1 Mty-1 is $0.4 billion per year.
Thus, the overall cost of the Mitchell bill with this SO2 emission reduction scenario would be about $6.5 billion per year with a probable bracket of $5-8 billion, not including the required capital cost. Friedman (14) estimated the cost of the Mitchell bill at $3.3 to $4.1 billion per year. Friedman based his figures on the so-called AIRTEST model developed for the U.S. EPA. No attempt is made here to unravel the
sources of the discrepancy between Friedman's estimate and the present analysis. Cost escalations and assumptions about the control technology mix may be the largest factors.
Given the substantial annual cost of proposed acid rain control programs, alternative strategies which emphasize the importance of reducing emissions near sensitive areas, intermittent control and other
techniques hold the promise of significant cost savings compared to the across-the-board continuous reductions exemplified by the Mitchell and Moynihan bills. Quantification of these costs will require a more detailed analysis than has been given here.
Receptor Mitigating Strategies
Lake liming. Liming of aquatic ecosystems has been a relatively longstanding practice for improving fish reproduction in slightly acidic lakes. Large scale programs for the liming of aquatic systems have been under way for some time in Sweden, and are now also being conducted in Norway, Canada, and New York state. The evidence to date suggests that the pH of lakes and streams can be raised to levels that will support fish. The long term effects are not yet established. The lime or
limestone mobilizes certain ions, such as aluminum ions, which can be detrimental in certain aquatic systems that have not previously been subjected to carbonate materials. In other aquatic systems there are substantial disruptions of the benthic communities. In addition, the release of the basic material is also difficult to predict, as lime clumps used to promote long-term releases occasionally become coated and ineffective.
The costs of liming can be inferred from the existing programs. By 1986 the Swedish liming program will encompass 200,000 acres of lakes at a cost of $40 million per year, or about $200 per acre of lake per year. Ontario's Ministry of Environment has successfully limed four lakes for about $50 per acre. In the Adirondack Mountains the cost of liming and restocking lakes with bass or trout is estimated to be in the range of
38
$30 to $400 per acre of lake per year, depending primarily on the accessibility of the lake.
Liming of lakes subject to acidification may be a necessary temporary measure to prevent irreversible damage while acidic deposition is being
reduced through emission control programs. As a permanent program for mitigating acid rain effects, however, it has many uncertainties and
drawbacks. From a purely chemical view, it would appear more efficient to neutralize the sulfur with lime at the source, rather than at the
receptor.
Soil liming. The use of lime to neutralize excess acidity in agricultural soils is a long established and successful practice.
However, agricultural crops are not as much endangered by acid rain as forests. Some experiments in Sweden showed that the application of lime to forest soils substantially neutralizes the soils, with one application lasting decades or even centuries. Whether or not this application can reduce the suspected stunting of forest groirth is not known, since these experiments have always been conducted in areas where substantial
concentrations of gaseous pollutants were also present. Also, it has not been determined from these experiments whether lime interrupts the acidic damage to fine roots or the mobilization of the minerals and metals.
Care must be taken in the application of lime so that the coniferous species, which require relatively acidic soils, will not be placed in too basic conditions, and that the nitrates, which are essential fertilizers, are not drawn away from these terrestrial ecosystems. It is known that relatively little of the basic material makes its way into the
surrounding aquatic systems, thus this would not be a cost-effective method of neutralizing lakes. Costs of liming forests are not known, but
Conclusions
Acid rain is difficult and expensive to control. Since vast quantities of both precursors of acid rain--SO2 and NOx--are emitted from a multitude of industrial, urban, and transportation sources,
restoring the rain acidity to what is postulated to be normal or
acceptable is probably not possible without major modifications to all existing emission sources. Modest reduction of the acid deposition rate in certain limited environmentally sensitive areas can be achieved most efficiently by partially curtailing emissions in regions and periods that can be shown to contribute the bulk of acidity to the sensitive areas. This curtailment in the source regions can be accomplished by (a) installing emission control devices on selected emission sources; (b) substituting low sulfur fuel for high sulfur fuel (however, this does not affect NOD emissions); and (c) importing electricity. All these can only be consi('ered partial remedies; ultimately acid rain will be more effectively ccntrolled by retiring the existing high emission sources and
replacing then. with new plants that will be equipped with advanced low emission combustion units.
References and Notes
1. D. Golomb, Atm. Environment (in press).
2. U.S.-Canada Memorandum of Intent on Transboundary Air Pollution, "Atmospheric Sciences and Analysis," Phase III Report, Environment Canada, Ottawa K1A1CB (1983).
3. J.N. Galloway and E.B. Cowling, J. Air Pollut. Control Assoc. 28, 229 (1978).
4. J.M. Omernik and C.F. Powers, "Total Akalinity of Surface Waters - a National Map," EPA-600/D-82-333, Corvallis, OR 97333 (1982).
5. U.S.-Canada Memorandum of Intent on Transboundary Air Pollution, "Impact Assessment," Interim Report Environment Canada K1A1CB (1981). 6. National Research Council of Canada, Subcommittee on Water,
"Acidification in the Canadian Aquatic Environment," Publications NRCC/CNRC, Ottawa, Canada, K1AOR6 (1981).
7. National Research Council, "Atmosphere-Biosphere Interactions: Toward a Petter Understanding of the Ecological Consequences of Fossil Fuel Combustion," Washington, DC 20418 (1981).
8. D.H. Pack, Science 208, 1143 (1980).
9. J.N. Galloway and D.M. Whelpdale, Atm. Environment 14, 409 (1980). 10. J.A. Garland, Atm. Environment 12, 349 (1978).
11. H. Rodhe, Ecol. Bull. 22, 123 (1976).
12. MAP3S/RAINE Research Community, Atm. Environment 16, 1603 (1982). 13. J.A. Fay and J.J. Rosenzweig, Atm. Environment 14, 355 (1980).
14. R.M. Friedman, "Testimony Before the Senate Committee on Environment and Public Works--Proposed Legislation (S. 1706 and S. 1709) Related to Acid Precipitation Control," Office of Technology Assessment, U.S. Congress, Washington, DC 20510 (1981).
15. G.S. Raynor and J.V. Hayes, Atm. Environment 16, 1647 (1982). 16. Congressional Record-Senate, January 26 (1983).
17. T.W. DeVitt and B.A. Laseke, Chem. Eng. Progress, May (1980). 18. K.E. Yaeger, Ann. Rev. Energy 5, 357 (1980).
19. R.A. Chapnian and M.A. Wells, "Coal Resources and Sulphur Emission Regulations," Teknekron Inc., Berkeley, CA 94704, Report to EPA, EPA-600/7-81-086 (1981).
20. This work was performed under the sponsorship of MIT's Center for Energy Policy Research whose support is gratefully acknowledged. Helpful data, ideas and advice were received from Drs. L.C. Cox, D.C. White, J.M. Deutch and J.M. Beer of MIT, and W.C. Labys of W. Virginia University.
